New Guideline: C-MUST-USE

[Havvy]

Basically, a guideline that states that any type that should be #[must_use]d should actually have #[must_use]. Likewise for functions, although #[must_use] on functions is currently unstable with a tracking issue (rust-lang/rust#43302). Though what we do in the Reference is just hold the PR in limbo until the feature gets stabilized.